<div id="controls">
  <button id="ioPolyfill">IntersectionObserver polyfill</button>
  <button id="roPolyfill">ResizeObserver polyfill</button><br />
  <label for="display">display</label>
  <select name="display" id="display">
    <option value="displayBlock">block</option>
    <option value="displayNone">none</option>
  </select>
  <label for="envHeight">envHeight</label>
  <select name="envHeight" id="envHeight">
    <option value="envHeightAuto">auto</option>
    <option value="envHeightHundred">100%</option>
    <option value="envHeight200">200px</option>
  </select>
  <label for="targetHeight">targetHeight</label>
  <select name="targetHeight" id="targetHeight">
    <option value="targetHeightAuto">auto</option>
    <option value="targetHeightHundred">100%</option>
    <option value="targetHeight200">200px</option>
  </select>

  <button id="start">start</button>
  <span>Detected changes: <span id="changes">0</span></span>
</div>
<div id="stage">
  <div>
    <div id="env">
      <div id="target">
        <div id="check"></div>
      </div>
    </div>
  </div>
</div>
